1. Field of the Invention
The present invention relates to a suspension bushing adapted to be used in a suspension system of a motor vehicle.
2. Description of Related Art
To prevent the transmission of vibrations in a suspension system of a motor vehicle, suspension bushings have been mounted in coupling portions between a vehicle body and a suspension arm. The suspension bushing generally has an arrangement that, as shown in FIG. 1, an inner cylindrical member 10 and an outer cylindrical member 12 are disposed coaxially, and a vibration-damping rubber member 14 is interposed between the inner and outer cylindrical members 10 and 12.
To improve the stability and controllability of a motor vehicle, it is preferable to increase the rigidity of the suspension bushing in the directions perpendicular to the axial direction (in the directions of arrows a in FIG. 1), and to ensure an improved ride of a motor vehicle, it is preferable to reduce the rigidity thereof in the torsional directions (in the directions of arrows b).
Japanese patent application laid-open No. Sho 64-30937, for example, discloses a suspension bushing, as shown in FIG. 1, of which the inner cylindrical member 10 includes an expanding portion 16 having a spherical configuration in an axially central portion thereof. With this arrangement, the thickness of the axially central portion of the vibration-damping rubber member 14 decreases to increase the rigidity in the directions perpendicular to the axial direction thereof, whereas the thickness of the left and right portions of the vibration-damping rubber member 14 do not decrease, thus ensuring reduction of the rigidity in the torsional directions to a relatively small level.
Recently, further improvement of the stability and controllability has been required. The above arrangement, however, could not satisfy this requirement.
The inner clindrical member has been generally prepared by forging or press forming. However, when the expanding portion is required to have a great height, for example, the expanding portion is difficult to form integrally with the inner cylindrical member. To overcome this problem, Japanese utility model application laid-open No. Hei 6-76729, for example, has proposed another suspension bushing, as shown in FIG. 2, of which an inner cylindrical member 18 has a depression 22 along an outer periphery of the axially central portion thereof. A cylindrical resin member 24 of which an outer periphery expands outwardly into a generally spherical configuration is press-fitted in the depression 22 to define an expanding portion. A vibration-damping rubber member 26 is interposed between an outer cylindrical member 20 and the inner cylindrical member 18 including the expanding portion.
This conventional arrangement, however, has a problem that the thickness of the inner cylindrical member 18 decreases partly due to the provision of the depression 22, thus reducing buckling strength of the inner cylindrical member 18. Furthermore, the cylindrical resin member 24 having a nonuniform thickness is difficult to form, because of shrinkage occurring during the molding operation, or the like. In addition, when the expanding portion is too high, it may receive a large load to cause falling-off or cracking thereof.